Science —

New insights into neutron stars

New observations from a pair of of space based observatories (XMM-Newton and …

The life of a star can be described as a battle between two forces, gravity and pressure. Once a thermonuclear fusion reaction has begun and is sustained in the heart of a star, it will push matter outward, constantly fighting gravity, which is trying to pull all the matter in the star inward. What wins out in this cosmological tug-of-war depends primarily on one thing: the mass of the star. If a star, or stellar remnant, has a mass that is within in a certain range—heavier than the Chandrasekhar mass, but below the Tolman-Oppenheimer-Volkoff limit—then it is likely that it will end its life as a neutron star.

Neutron stars are clusters of mass that are between 1.5 and 3 times the mass of the sun, but are much, much smaller, often with diameters on the order of tens of kilometers. Given this, they represent the densest material known.

Prof. Farnsworth: One pound of it weighs 10,000 pounds!

According to Sudip Bhattacharyya at NASA’s Goddard Space Flight Center, "[Neutron stars represent] fundamental physics. There could be exotic kinds of particles or states of matter, such as quark matter, in the centres of neutron stars, but it’s impossible to create them in the lab. The only way to find out is to understand neutron stars."

To further our knowledge of neutron stars, scientists need to measure two key properties: their size, and their mass. A pair of recent studies using the ESA's XMM-Newton and JAXA/NASA's Suzaku X-ray observatories have made great strides in advancing our understanding of neutron stars. Using XMM-Newton, Bhattacharyya and colleague Tod Strohmayer studied the binary star system Serpens X-1. They looked at the spectral line that results from hot iron atoms whirling around the star at a speed of about 0.4c. While they are not the first to examine this phenomenon, they were able to do it with a much higher resolution thanks to XMM-Newton's large mirrors. They found that the iron line was smeared by the doppler effect, relativistic beaming, and the extreme curvature of space near such a massive object. According to Strohmayer, "We have seen these asymmetric lines from many black holes, but this is the first confirmation that neutron stars can produce them as well. It shows that the way neutron stars accrete matter is not very different from that of black holes, and gives us a new tool to probe Einstein’s theory."

In a concurrent study using the Suzaku X-ray observatory, the same authors teamed with researchers from the University of Michigan to study a trio of neutron star binary systems. Measurements from the two observatories put the diameter of these massive (1.5-3 times the mass of the sun) objects at only 29 to 33 km. With the new-found ability to measure the relativistic iron line about neutron stars, astronomers have another tool to add to their arsenal when it comes to studying these exotic bits of matter. With an estimate of size and mass, physicists can generate an equation of state for the matter packed inside these cosmic bodies. This new technique could also open up new avenues of research, probing the interior of the stars.

Ars Science Video >

Apollo: The Greatest Leap

In honor of the 50th anniversary of the beginning of the Apollo Program, Ars Technica brings you an in depth look at the Apollo missions through the eyes of the participants.

Apollo: The Greatest Leap

Apollo: The Greatest Leap

In honor of the 50th anniversary of the beginning of the Apollo Program, Ars Technica brings you an in depth look at the Apollo missions through the eyes of the participants.

Matt Ford
Matt is a contributing writer at Ars Technica, focusing on physics, astronomy, chemistry, mathematics, and engineering. When he's not writing, he works on realtime models of large-scale engineering systems. Emailzeotherm@gmail.com//Twitter@zeotherm